Polymorphism Why use List list new ArrayList instead of ArrayList list new ArrayList

Question

Possible Duplicate    Why should the interface for a Java class be prefered        When should I use   List lt Object gt  list   new ArrayList lt Object gt       ArrayList inherits from List  so if some features in ArrayList aren t in List  then I will have lost some of the features of ArrayList  right  And the compiler will notice an error when trying to access these methods

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I use that construction whenever I don t want to add complexity to the problem  It s just a list  no need to say what kind of List it is  as it doesn t matter to the problem  I often use Collection for most of my solutions  as  in the end  most of the times  for the rest of the software  what really matters is the content it holds  and I don t want to add new objects to the Collection   Futhermore  you use that construction when you think that you may want to change the implemenation of list you are using  Let s say you were using the construction with an ArrayList  and your problem wasn t thread safe  Now  you want to make it thread safe  and for part of your solution  you change to use a Vector  for example  As for the other uses of that list won t matter if it s a AraryList or a Vector  just a List  no new modifications will be needed

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The main reason you d do this is to decouple your code from a specific implementation of the interface  When you write your code like this   List list   new ArrayList        the rest of your code only knows that data is of type List  which is preferable because it allows you to switch between different implementations of the List interface with ease   For instance  say you were writing a fairly large 3rd party library  and say that you decided to implement the core of your library with a LinkedList  If your library relies heavily on accessing elements in these lists  then eventually you ll find that you ve made a poor design decision  you ll realize that you should have used an ArrayList  which gives O 1  access time  instead of a LinkedList  which gives O n  access time   Assuming you have been programming to an interface  making such a change is easy  You would simply change the instance of List from   List list   new LinkedList      to  List list   new ArrayList        and you know that this will work because you have written your code to follow the contract provided by the List interface    On the other hand  if you had implemented the core of your library using LinkedList list   new LinkedList    making such a change wouldn t be as easy  as there is no guarantee that the rest of your code doesn t make use of methods specific to the LinkedList class   All in all  the choice is simply a matter of design    but this kind of design is very important  especially when working on large projects   as it will allow you to make implementation-specific changes later without breaking existing code

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This is also helpful when exposing a public interface  If you have a method like this   public ArrayList getList      Then you decide to change it to   public LinkedList getList      Anyone who was doing ArrayList list   yourClass getList   will need to change their code  On the other hand  if you do   public List getList      Changing the implementation doesn t change anything for the users of your API

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In general you want to program against an interface  This allows you to exchange the implementation at any time  This is very useful especially when you get passed an implementation you don t know   However  there are certain situations where you prefer to use the concrete implementation  For example when serialize in GWT

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This is called programming to interface  This will be helpful in case if you wish to move to some other implementation of List in the future  If you want some methods in ArrayList then you would need to program to the implementation that is ArrayList a   new ArrayList

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I think  tsatiz s answer is mostly right  programming to an interface rather than an implementation   However  by programming to the interface you won t lose any functionality  Let me explain    If you declare your variable as a List lt type gt  list   new ArrayList lt type gt  you do not actually lose any functionality of the ArrayList  All you need to do is to cast your list down to an ArrayList  Here s an example   List lt String gt  list   new ArrayList lt String gt       ArrayList lt String gt   list  ensureCapacity 19     Ultimately I think tsatiz is correct as once you cast to an ArrayList you re no longer coding to an interface  However  it s still a good practice to initially code to an interface and  if it later becomes necessary  code to an implementation if you must   Hope that helps

User · Answer

I guess the core of your question is why to  program to an interface  not to an implementation   Simply because an interface gives you more abstraction  and makes the code more flexible and resilient to changes  because you can use different implementations of the same interface in this case you may want to change your List implementation to a linkedList instead of an ArrayList   without changing its client

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This enables you to write something like   void doSomething         List lt String gt list   new ArrayList lt String gt           do something     Later on  you might want to change it to   void doSomething         List lt String gt list   new LinkedList lt String gt           do something     without having to change the rest of the method   However  if you want to use a CopyOnWriteArrayList for example  you would need to declare it as such  and not as a List if you wanted to use its extra methods  addIfAbsent for example    void doSomething         CopyOnWriteArrayList lt String gt list   new CopyOnWriteArrayList lt String gt           do something  for example      list addIfAbsent  abc

[java] Polymorphism: Why use "List list = new ArrayList" instead of "ArrayList list = new ArrayList"?

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